Method for preparing pre-form materials

ABSTRACT

A pre-form and a method of preparing pre-forms are provided. The pre-forms comprise a resin and at least three layers of oriented fibre tows. The pre-forms comprise fibre tows instead of the traditional prepregs to enhance rearranging of resin and/or fibres during subsequent processing as well as provide greater freedom, a price reduction and/or a reduction of waste. The pre-forms may be formed three-dimensionally to enhance coupling to further pre-forms or other structures and/or to enhance shaping of the pre-form to a final three-dimensional shape. The method of preparation of pre-forms involves providing an adhesive between layers of fibres and providing a resin in contact with at least one of the layers of fibres. The resin is preferably provided in a non-continuous layer to allow for removal of gas at least partially in a direction orthogonal to the layers of resin. Advantageously, during manufacturing, the fibres are placed with an exact start and ending location, thereby avoiding waste. The pre-forms are suitable for preparation of composite structures like for example spars for wind turbine blades.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.10/547,749, a National Stage Application of International ApplicationPCT/EP03/002293 filed Mar. 6, 2003, herein incorporated by reference.

FIELD OF THE INVENTION

The invention relates to fibre-reinforced composites. In particular, theinvention relates to a semi-manufacture comprising a resin and severallayers of fibre tows.

BACKGROUND OF THE INVENTION

Pre-forms comprising resin and fibres are known in the prior art.

U.S. Pat. No. 6,139,942 discloses a pre-form with a stack of partiallyimpregnated fabric and un-impregnated fabric. The layers of such a stackmay slide before curing and may hence be difficult to handle. It issuggested to use cross-ply stitching to prevent this. However, thisprocess is tedious and may introduce undesirable restrictions to theshape of the stack during curing.

EP 0 475 883 also discloses a pre-form with a plurality of orientedfibre layers. However, the pre-form requires infusion of a resin to curewhich may be time-consuming and to some extent prevent rearranging ofthe fibres and resin during curing.

WO 02/090089 discloses a moulding material having a ventilatingstructure in the otherwise continuous resin layers. The ventilatingstructure is designed to allow gas to be removed from the mouldingmaterial during processing in the plane of the resin and/or in the planeof the reinforcement material. As the ground plan of the mouldingmaterial increases, this will become a still less safe way to remove gasfrom the moulding material due to the increasing risk of plugging duringprocessing.

EP 0 033 224 concerns a material for forming fibre reinforced structureswith a thermoplastic resin. The thermoplastic resin is provided bypassing the fibres through a bath with thermoplastic resin or by mixingdry fibres with thermoplastic fibres and heat the collection From thesecoated fibres is formed a fabric by weaving or knitting. The descriptionspecifically teaches away from the use thermosetting resins.

EP 1 145 541 concerns a material consisting of “fibre bundle layers.”Each fibre bundle layer consists of apparently fully wetted fibrebundles and thermosetting resin and the individual layers are orientedso that the fibres of different layers are oriented differently.

DE 198 09 264 concerns a material consisting of a number of fibrelayers, each layer having an individual orientation, and a meltingadhesive. The preferred and only described melting adhesives arethermoplastic adhesives (“Schmelzklebereblide aus thermoplastischemPolymermaterial”, page 3, lines 3-7).

SUMMARY OF THE INVENTION

It is the object of an embodiment of the invention to provide a pre-formthat may be used for manufacture of reinforced composites and whichposes good reproducibility, low porosity and good physical properties.

It is another object of an embodiment of the invention to provide amethod for pre-consolidating a pre-form involving a partial curing of aresin.

It is a further object of an embodiment of the invention to provide apre-form and a method of production of a pre-form that are adaptable toautomated processing.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross sectional view of a pre-form.

FIGS. 2A-2J are schematic views of preferred geometrical ground plansand fibre orientations in a pre-form, wherein FIG. 2A is a rectangularpre-form and FIGS. 2B-2F are pre-forms which have substantiallytrapezoidal ground plans, FIG. 2G is a pre-form with a triangular groundplan, FIG. 2H is a pre-form with an irregular quadrilateral ground plan,FIG. 2I is a layer having two main fibre orientations, and FIG. 2J is apre-form having a layer with non-straight fibres.

FIGS. 3A-3D are schematic views of preferred configurations of a resinlayer, wherein FIG. 3A is an oriented pattern with lines of resinorthogonal to a main fibre orientation, FIG. 3B is the resin distributedto form a pattern of solid or semisolid particles, FIG. 3C is a randomresin pattern, and FIG. 3D is a non-continuous layer of resin furthershowing a number of through-going holes.

FIGS. 4A and 4B are schematic views of preferred configurations of anadhesive, in accordance with the present invention, wherein, in FIG. 4A,the adhesive is provided as strips and FIG. 4B has the adhesive appliedin a continuous line forming a zigzag pattern.

FIG. 5A is a plan view of a pre-form with tapered parts, FIG. 5B is across-sectional view of the pre-form taken along line I-I in FIG. 5A,and FIG. 5C is a cross-sectional view of the pre-form taken along lineII-II in FIG. 5A.

FIG. 6 is a schematic view of a preferred method of preparing a resin,in accordance with the present invention.

FIG. 7 depicts a pre-form having tapered parts prepared from fibrelayers having substantially the same size, in accordance with thepresent invention.

FIG. 8 is a pre-form enhanced for coupling of two composite memberscomprising two different types of reinforcement fibres.

DETAILED DESCRIPTION

According to one aspect of the present invention, there is provided apre-form comprising a resin and at least three layers of oriented fibretows, wherein the resin is mainly a thermosetting resin, and a part ofsaid pre-form being formed three-dimensionally so that said part of saidpre-form is tapered.

According to another aspect of the present invention, there is provideda method for preparing a pre-form comprising the steps of: providinglayers of oriented fibre tows, providing an adhesive between said layersof fibres to at least partially immobilizing the fibres, providing aresin in contact with at least one of the layers of fibre tows, whereina fibre layer is provided off the edge of a preceding fibre layer,thereby realizing a tapered part of the pre-form and the resin is athermosetting resin.

In accordance with another aspect of the present invention, there isprovided a method for preparing a pre-form comprising the steps ofproviding an adhesive between layers of fibres to at least partiallyimmobilizing the fibres providing a resin in contact with at least oneof the layers of fibre, wherein the resin is provided to formnon-continuous layer or layers.

In accordance with yet another aspect of the present invention, there isprovided a method for preparing a pre-form comprising the steps ofproviding an adhesive between layers of fibres to at least partiallyimmobilizing the fibres providing a resin in contact with at least oneof the layers of fibre, wherein the fibres are provided to form at leastone of a desired orientation and form of ground plan by selectively atleast one of initiating and terminating fibres.

A pre-form is a composite material comprising fibres and—unlessotherwise stated—an uncured resin. The fibres are preferably provided inlayers of oriented fibres like for example individual fibres, fibretows, fibre tow-pregs or prepregs. Individual fibres, fibre tows andfibre tow-pregs are advantageous over prepregs, since the individualfibres are less bounded and hence may rearrange easier during subsequentprocessing. Furthermore, individual fibres, fibre tows and tow-pregs areadvantageous over prepregs in that they may be provided in the pre-formwith a greater freedom, the price is lower as well as the amount ofwaste may be lower. The invention provides a pre-form comprising a resinand at least two layers of oriented fibre tows, however, the advantageof using a pre-form or a method according to the present invention willincrease as the number of layers of oriented fibre tows are increased.Hence, the pre-form preferably comprises at least three layers oforiented fibre tows. A higher number of layers like, e.g., 4, 5, 8, 10,15, 20, 50, 100 or more layers may be used within the scope of theinvention.

Besides fibres and resin, a pre-form according to the invention may forexample contain one or more of fillers (e.g., a cheap inert material)and/or solvents and/or diluents and/or rheological agents and/orviscosity adjusting agent.

The layers of oriented fibres are fibre tows or tow-pregs contrary toprepregs, since this provides a higher degree of freedom of design andwill allow for a lower viscosity and mobility of fibres duringsubsequent processing of a pre-form, e.g., pre-consolidation or curing.Furthermore, pre-forms prepared from fibre tows and tow-pregs areadvantageous over pre-forms prepared from prepregs in that the cost ofproduction is lower as well as the amount of waste is typically lower.Fibre tows are bundles of a large number of individual fibres, e.g.,1,000's, 10,000's or 100,000's of fibres. Tow-pregs are at leastpartially impregnated fibre tows.

It may be theorized that the strength of a composite depends amongstothers on the strength of the interface between the fibres and thematrix material (i.e. the cured resin). As the stiffness of the fibre isincreased, the sensitivity to the strength of the interface is alsoincreased. Presence of porosity may weaken the interface but the actualeffect of the porosity depends for example on the positioning and thesize of the pores. Generally speaking, the greater the pores and thegreater the amount of pores, the worse. Another aspect is the wetting ofthe fibres. The difficulty in getting a good wetting of the fibresincreases as the fibre diameter is decreased. The processes and productsof the present invention are particularly advantageous for pre-formscomprising thin and stiff fibres like for example carbon fibres,however, these processes and products are also superior to the prior artwhen other types of fibres are used as reinforcement like for exampleglass fibres, aramid fibres, synthetic fibres (e.g., acrylic, polyester,PAN, PET, PE, PP or PBO-fibres), bio fibres (e.g., hemp, jute, cellulosefibres etc.), mineral fibres (e.g., ROCKWOOL), metal fibres (e.g.,steel, aluminium, brass, copper, etc.) or boron fibres.

Traditionally, gas enclosed in the pre-form prior to and during curinghas traditionally been removed in the direction of the fibres, i.e. inthe plane of a resin layer. Hence, the larger the structure, the longerthe gas has to travel to be released from the structure. The risk thatgas becomes trapped inside a cured structure is hence increased with thesize of the structure. It appears that the problem with entrapped gas isparticularly pronounced when the reinforcement is unidirectional fibres.It may be speculated that this is due to the very close packing of thefibres, which may arise in some areas of a composite reinforced byunidirectional fibres. However, problems concerning entrapped gas mayalso be present in other types of fibre orientations, e.g., biaxial orrandom orientations and the inventive idea of the present invention ishence advantageous for any type of fibre orientation even if theadvantage is greatest when using a unidirectional fibre orientation.

By gas is herein meant entrapped atmospheric air as well as gaseousproducts, byproducts and starting materials related to the preparationprocess.

The fibres may be a mixture of more than one type of fibres. Forexample, a combination of glass fibres and carbon fibres may be used,but any combination of two or more of the fibre types mentioned hereinare feasible. The mixture may be homogeneous, with differentconcentrations in separate fibre layers or with different concentrationsof fibres within any fibre layer. Mixing of fibres may be advantageous,since this opens for tailoring of material properties, for example froma combined stress/cost-perspective, or parts of a pre-form particularlysuitable for connecting to other materials may be provided. However, ina preferred embodiment, the fibres are primarily or exclusively carbonfibres.

By carbon fibres is hereinafter meant fibres where the main component iscarbon. Hence, by this definition carbon fibres comprise fibres withgraphite, amorphous carbon or carbon nano-tubes. Thus, carbon fibresproduced via for example a polyacrylonitril-route and a pitch-basedroute are comprised by this definition.

By fibres are hereinafter meant particles having an aspect ratio(length/equivalent diameter) of more than 10. By equivalent diameter ismeant the diameter of a circle having the same area as the crosssectional area of the particle. However, in a preferred embodiment, thefibres are continuous fibres, i.e. fibres that substantially run fromone edge of a pre-form to another.

The resin may be a thermoplastic or a thermosetting resin, however it ispreferred to use a thermosetting resin for reasons of chemical andthermal stability as well as ease of processing. It is further preferredthat the resin is an epoxy-based or a polyester-based resin, mostpreferably an epoxy-based resin. The resin may comprise more than oneresin system. It may be advantageous to use more than one resin systemto be able to optimise the properties of the resin for the subsequentsteps of processing, for example with respect to viscosity andtiming/controlling of the curing process. These systems may or may notbe based on the same type of resin, however, it is preferred that suchsystems are based on the same type of resin like for example two or moreepoxy-based systems. In another preferred embodiment, the resin typesdiffer but the resins are compatible.

The method according to the invention is adapted to automatedprocessing. For example, the layers of oriented fibre tows, the adhesiveand the resin may advantageously be distributed by a robot. Theautomation is facilitated by an at least partial immobilization offibres by an adhesive, which will prevent or at least greatly reducedisturbance in the layers of oriented fibre tows. When the adhesive isonly applied to selected areas of the ground plan of the pre-form, timeis furthermore saved compared to distribution of resin over the entireground plan.

Resin systems may contain components, which may be irritant or harmfulwhen in contact with naked skin, if ingested or inhaled. Avoidance ofdirect contact is therefore highly desirable. Since the products andprocesses according to the invention are particularly well suited forautomation, the products and processes according to the inventionrepresent a significant improvement to the working environment.

In FIG. 1 an example of a schematic sectional view of a pre-form isshown indicating an example of the order of the components. In a realpre-form, the distance between the layers would be much smaller and theresin and adhesive would usually be partially absorbed into the layersof fibres. Layers of oriented fibre 2 are provided with strips ofadhesive 6 (see FIG. 4) at an angle—here about orthogonal—to the layersof oriented fibre tows 2. Two layers of resin 4 are also provided. Theresin 4 is distributed as a number of lines (see FIG. 3) at anangle—here about orthogonal—to the layers of oriented fibre tows 2. Theresin hence is distributed in a non-continuous layer to allow for gas toescape from the pre-form orthogonal to the direction of the fibre tows.

In FIG. 1 the resin is provided between two layers of fibre tows. Thisis the preferred positioning of resin and when this positioning is used,it is highly desirable that the resin is distributed in non-continuouslayers. However, the resin may also be provided in contact with only onelayer of fibre tows, i.e. in the top or at the bottom of the pre-form.In this case, it is preferred to provide the resin at the bottom of thepre-form and the resin may be provided in a continuous layer as gas willnot usually have to escape through the resin layer. In a preferredembodiment, resin is only provided at top and/or the bottom of thepre-form, i.e. only adhesive is provided between the layers of fibres.In another preferred embodiment, resin is only provided at the bottom ofthe pre-form, i.e. between the layers of fibres only adhesive isprovided.

The adhesive 6 should at least partially immobilize the fibres that areprovided on top of the adhesive. The adhesive may be any type ofadhesive, however, the adhesive should be compatible to the resin,preferably the adhesive is a resin-type adhesive and related to theresin of the pre-form in the sense that it comprises the same type ofchemistry. For example, the adhesive may comprise at least one of thecomponents of the resin (e.g., a common epoxy component). One way toensure compatibility between the resin and the adhesive is to usesubstantially the same composition. In a preferred embodiment, thecomposition of the adhesive is the same as the composition of theadhesive. It is within the scope of the invention to use more than oneadhesive in a pre-form. For example some portions of adhesive may havethe same composition as the resin, whereas other portions may have adifferent composition.

Examples of preferred embodiments of geometrical ground plans 10 ofpre-forms are shown in FIG. 2. The lines 2 indicate the main fibreorientation(s) of a fibre layer. Each layer of fibres typicallycomprises a large number (e.g., multiple millions) of fibres oriented inthe main and optionally further orientations. A person skilled in theart will be able to derive a number of other geometrical ground planswithout deriving from the inventive idea of the invention. FIG. 2A showsa rectangular pre-form, which may be particularly suitable for flat orcylindrical structures. FIG. 2B-FIG. 2F show pre-forms withsubstantially trapezoid ground plans. The angles α and β may be the sameor different, however, it is preferred that these angles aresubstantially the same, since the pre-form may then be used forproduction of for example conical structures.

The ratio of the distance between the parallel sides and the length ofany of the parallel sides is preferably at least 3 and more preferablyat least 5, since such pre-forms are particularly useful for productionof long, slightly conical structures like spars for wind turbine blades.The ground plan in FIG. 2F has a ratio of about 6.

FIG. 2G shows a pre-form with a triangular ground plan. Again, it ispreferred that the angles α and β are substantially the same. Such apre-form may be particularly useful for reinforcing a structure with arelatively sharp corner. The pre-form in FIG. 2H has a more irregularquadrangular ground plan. Such quadrangular ground plans may or may nothave one or two angles of 90°.

In FIG. 2I is an example of a layer having two main fibre orientations.Here, the fibres are mainly oriented parallel to the longer edges,however, other orientations are feasible as well as more than two mainorientations. FIG. 2J shows an example of a layer having non-straightfibres. The fibres are preferably oriented to optimise the finalstructure—after shaping and curing—with respect to strength and/or otherproperties.

The fibres 2 may be provided in any desirable orientation like forexample unidirectional, biaxial or random. However, the fibres should beoriented as to strengthen areas of the final structure, which will beexposed to a higher stress during service. Typically, this may berealized by orienting the fibres primarily unidirectionally and eithersubstantially parallel to or orthogonal to an edge of the pre-form. InFIG. 2A, C, D, H and I the fibres are placed substantially parallel toat least one edge of the pre-form and in FIGS. 2A, B, E, F, G and H thefibre tows are placed substantially orthogonal to at least one edge ofthe pre-form. If the ground plan has two parallel sides and the ratio ofthe distance between the parallel sides and the length of any of theparallel sides is very large, i.e. >5, then unidirectional fibres placedbetween the two parallel sides may be considered to be substantiallyparallel to the longer edges (see, e.g., FIG. 2F). Other ways oforientating the fibres may be derived by a person skilled in the artwithout departing from the inventive idea of the invention.

The orientation of fibres may or may not be the same in all the layersof fibres; however, in a preferred embodiment the fibres are orientedsubstantially the same in all the layers of fibres. When one or morelayers of fibres are oriented in another way as other layers, this mayfor example be if the stress analysis suggests a multi-axial fibreplacing, but unidirectional fibre placing is favorable for manufacturingreasons.

Another way to strengthen the area of the final structure, which will beexposed to higher stress during service, is to increase the amount offibres in these areas. An example of this is shown in FIG. 2E, where thearea near the central part of the pre-form has a greater amount of fibretows than the outer parts of the pre-form.

It is preferred that the resin is provided to form a non-continuouslayer or layers, even if this is not a requirement for layers of resinwhere gas will not escape through during a subsequent consolidationand/or curing of the pre-form. The resin is preferably semisolid and maystick to and/or at least partially immobilize fibres of one or morelayers. In a preferred embodiment, the resin is distributed to form apattern of solid or semisolid particles 4 b, these particles may forexample be sprinkled over a layer of fibres as shown in FIG. 3B. Thediscrete dots may alternatively for example be formed from a resinprovided as a liquid. A liquid resin may also be provided as a line orseveral lines, which maybe form an oriented pattern, a random pattern ora combined pattern. An example of an oriented pattern is shown in FIG.3A, where a resin is distributed as lines of resin 4 a orthogonal to amain fibre orientation. As a variation to the distribution shown in FIG.3A, the resin may be provided partially over the edge, i.e. the turningpart of the string may be off the ground plan to provide for a more evendensity of resin. However, this will result in an undesired waste andshould be omitted for example by controlling the flow of the resinduring the applying. An example of a random pattern is shown in FIG. 3C,where a resin is distributed as curly lines. A different approach to anon-continuous layer of resin is shown in FIG. 3D where a sheet of resin4 d having a number of through-going holes 12 are provided. As it isobvious from these examples of resin patterns, a person skilled in theart will be able to provide other patterns without departing from theinventive idea of the invention.

The adhesive 6 may in principle be provided in the same patterns as theresin. However, it is preferred to provide a less dense pattern for theadhesive to save time. In FIG. 4 some preferred embodiments of theadhesive distribution are shown. It is important to keep in mind thatthe purpose of the adhesive is to ensure that the fibres are at leastpartially immobilized to facilitate fibre laying. Furthermore, theadhesive will often increase the mechanical strength and hence thehandleability of an unconsolidated and uncured pre-form by at leastpartially fixing adjacent layers of fibre to each other. One way toensure a facilitation of the fibre laying is to provide a strip ofadhesive close to or exactly where the fibres are initiated during fibrelaying. In FIG. 4A the direction of the fibre laying is indicated by thearrow 14. Hence, the fibres are initiated near the adhesive 6 a. Apreferred way to ensure a relatively good fixation of the fibres is toprovide adhesive 6 b near the termination of the fibres. If the adhesive6 a and 6 b does not provide sufficient fixation of the fibres, furtheradhesive 6 c may be provided. In FIG. 4A the adhesive is provided asstrips, however, other embodiments are also feasible like for exampledots, broken or curved lines, etc. In some cases, automation may favoran adhesive pattern, where the adhesive is applied in a continuous line,for example as a zigzag pattern as shown in FIG. 4B. This is an exampleof a pattern where the number of adhesive initiations and terminationsare reduced compared to the pattern in FIG. 4A. A person skilled in theart will appreciate the advantage of providing only a limited amount ofadhesive compared to either a full or nearly full layer of resin orcross-ply stitching, particularly with respect to the time saved duringprocessing and ease of automation.

In some applications, the pre-form is intended for reinforcingstructures with a non-circular cross section like for example a sparhaving a substantially rectangular cross section where the pre-formshould be bent around a relatively sharp edge. The preferred orientationof fibres in such case is that the main fibre orientation is parallel tothe edge, e.g., in the direction I-I in FIG. 5B. It may then beadvantageous to form at least part of the pre-form three-dimensionallyto enhance shaping of the pre-form. To realize a significant outcome ofa three-dimensional forming, the pre-form usually should preferablycomprise at least three layers of oriented fibre tows, since if thepre-form consists of only two layers, the pre-form may usually be bentwithout three-dimensional shaping of the pre-form. The higher the numberof layers, the greater the benefit from three-dimensionally forming ofthe pre-form. In a preferred embodiment, a pre-form is provided with twotapered sections 22 towards the edges substantially parallel to thefibre orientation as indicated in FIGS. 5A-5C; however, a person skilledin the art may derive a number of variations without deriving from theinventive idea of the invention. Such variations may for example beusing one, three or another number of tapered parts, using one or moresteps instead of tapered parts, placing a tapered part away from anedge, e.g., near the center, etc.

By being formed three-dimensionally is herein meant that the thickness(e.g., the number of layers or amount of fibres and/or resin) and/or theform of the ground plan is adjusted for a part (e.g., 20, 22) of thepre-form relative to the bulk (e.g., 24) of the pre-form.

Three-dimensional forming may also be applied for reducing of theinterfacial stress between a pre-form and an adjacent structure.Examples of such adjacent structures are other pre-forms and parts oflarger structures, e.g., a root of a blade for a wind turbine.Typically, such three-dimensional forms will involve the creation of alarge contact area orthogonal to the main stress direction in at leastone axis. In FIG. 5A a part 20 is an example of a three-dimensional formfor reducing the interfacial stress between the pre-form and an adjacentconnected structure. A distance much greater than the orthogonaldistance separates the terminations of the layers of fibres in thetapered section 20 as seen in the cross section along I-I in FIG. 5B andhence the interfacial stress will be reduced.

A particular ground plan or three-dimensional form may for example berealized by selective initiating and/or terminating fibre tows duringfibre laying.

The main function of the adhesive is to immobilize the fibres as theyare placed on top of the adhesive. This can be achieved by having atacky adhesive, whereby the fibres stick to the tacky adhesive. Theadhesive may be any tacky material, or a solid with a tacky surface andthe adhesive may for example comprise polyester, polyurethane, epoxy orsimilar compounds or a combination of these. It is within the scope ofthe invention to use any material or combination of materials having atacky surface including solid materials with tacky surfaces. More thanone type of adhesive may be used within a pre-form. For example, it iswithin the scope of the invention to use the resin as an adhesivebetween the layers of fibre tows where a resin is provided or to use asecond type of resin below the first layer of fibre tows.

The resin may be liquid, semisolid or solid material. The resin may forexample be based on unsaturated polyester, polyurethane, epoxy,thermoplastics or similar chemical compounds, including combinations ofthese.

In a preferred embodiment of the invention, the resin is a liquid andthe resin is introduced by Resin Transfer Moulding, RTM, or VacuumAssisted Resin Transfer Moulding, VARTM, into an entity comprisingseveral layers of oriented fibre tows, which were previously immobilizedduring fibre laying by an adhesive.

In another preferred embodiment, the resin is a solid. An entitycomprising several layers of oriented fibre tows, which were previouslyimmobilized during fibre laying by an adhesive, and a solid resin systemis heated under vacuum in order to prepare a pre-consolidated pre-form.

In a further preferred embodiment, the resin is a semisolid andfunctions both as resin and as adhesive, i.e. during fibre laying, theresin will immobilize the fibres and during subsequent processing, itfunctions as a matrix material.

The resin may comprise more than one system, for example the resin maycomprise two systems or even more systems. These systems may be anycombination of different or the same type of systems. However, it ispreferred that the resin comprises two substantially epoxy-basedsystems. In a preferred embodiment, two epoxy-based systems comprise acommon component. The common component may for example be a commoncatalyst, a common amine component or a common epoxy component. However,it is preferred that the common component is an epoxy component. A resincomprising two epoxy-based systems with a common epoxy component maycomprise an amine-component of a first epoxy-based system that willreact to the common epoxy component at a first relatively lowtemperature, like for example below 50° C., preferably about roomtemperature. At this first temperature, a second epoxy-based system ispreferably non-reactive or the reaction takes place at a very low rate.Since the reaction rate of the second epoxy-based system should be verylow, it may advantageously be catalysed by a catalyst, which isnon-active until activated. This activation may for example be byUV-light, addition of a compound or by heating. However, it is preferredthat the catalyst is activated by heating.

In one embodiment sketched in FIG. 6, a pre-mix 36 comprises aminecomponents 30 a and 30 b and a catalyst 32, preferably for catalysingthe curing of a second epoxy-based system. The pre-mix should be astable solution or slurry and if the viscosity is too low to preventprecipitation of a solid component like, e.g., a catalyst, a smallamount of an epoxy component, preferably a common epoxy component of thesystems, may be added. Typically 0.1 to 5% by weight of epoxy should besufficient to adjust the viscosity. The pre-mix and a common epoxycomponent should be mixed immediately before distribution of the resin40. The resin may be heated to decrease the viscosity, but preferablythe resin is semisolid at room temperature.

Resins to be used according to the present invention may be prepared inmost of the traditional ways familiar to a person skilled in the art andthe embodiment with regard to preparation of the resin that is disclosedin FIG. 6 should be considered as an example on how a resin may beprepared. This embodiment should by no means be regarded as a limitationof the scope of the invention.

Alternatively, a simple three-dimensional shape may be provided as shownin FIG. 7, where a number of identical ground plans of oriented fibres50 are placed on top of each other but shifted slightly. The lines shownon the ground plans 50 do not indicate the orientation of the fibre towsbut are merely included to enhance distinguishing the different layerswhen they are combined. In the middle section of FIG. 7, the layers areplaced on top of each other step by step and hence forming a pre-form inthe bottom part of FIG. 7 having parts 52 with a low number of layers offibres, parts 54 with an intermediate number of layers of fibres and apart 56 with a high number of layers of fibres. When a higher number oflayers are provided, then the parts 52 and 54 may be prepared to appearalmost taper-like. This method may simultaneously provide a pre-formhaving two, three, four or more tapered parts.

The properties of a fibre-reinforced composite depend to a large extenton the properties of the fibre. However, the properties of differenttypes of fibres vary considerably. For example, the coefficient ofthermal expansion of carbon fibres is very low, and in some cases evennegative. It may therefore be very difficult to connect carbonfibre-reinforced composites to composites reinforced by other types offibres and a pre-form comprising carbon fibres may thereforeadvantageously be enhanced for coupling to a composite member comprisinga second type fibres and a resin. Examples of second type fibres areglass fibres, aramid fibres, synthetic fibres (e.g., acrylic, polyester,PAN, PET, PE, PP or PBO fibres), bio fibres (e.g., hemp, jute, cellulosefibres, etc.), mineral fibres (e.g., ROCKWOOL), metal fibres (e.g.,steel, aluminium, brass, copper etc.) or boron fibres.

In a preferred embodiment, the pre-form is enhanced for connecting byproviding the pre-form with second type fibres. These second type fibresshould extend beyond the pre-form to provide a part for connection. Thesecond type fibres as well as the carbon fibres may be provided ininterlaced layers having an appreciable amount of the respective fibres.For example, the layers may exclusively have only one of the type offibres. In a preferred embodiment, the layers comprising second typefibres are provided as prepregs. The prepregs may be unidirectionalprepregs. However, experimental results suggest, surprisingly, thatbiaxial prepregs comprising the second type fibres provide a betterbasis for connecting of the pre-form to a structure reinforced by secondtype fibres.

Near the end of a layer of fibres interlaced in a material havingdifferent properties, a stress concentration will build up. To reduce orprevent coupling of stress from the ends of one layer to the ends of thenext layer, the distance of the interlace should be greater that theextent of the stress concentration. Since the extent of the stressconcentration is difficult to establish, it is preferred to use a safetymargin and hence separate the ends of two adjacent layers by at least 2times the extent of the stress concentration.

It is also reasonable to ensure that the distance between the nearestlayer end of the same type of fibre should be separated by a distancecorresponding to the extent of a stress concentration, preferably with asafety margin and hence using a factor of 2.

The extent of the stress concentration depends on a number of factors,like for example thickness of the layers, the type of fibres, the typeof resin, etc., and may be established by modelling or by empiricalmethods.

Pre-forms according to the invention and provided by a method accordingto the invention are very useful for pre-consolidation, since the escapeof gas from the pre-form is facilitated by the presence ofnon-continuous layers of resin. The pre-forms may alternatively be useddirectly for preparation of composite members by curing. Particularly,the pre-forms are highly useful for preparation of members for windturbine blades, since composites fulfilling the quality requirements andreproducibility requirements may be produced.

When larger structures comprising pre-forms according to the inventionor pre-forms produced by a method according to the invention are to beprepared, this may follow a method wherein the pre-form is shaped atleast partially plastically. The pre-form may be connected to furtherpre-forms before or after shaping to provide a larger structure. Thepre-form may also be connected to other structures. It is preferred butnot required that the connections involve a tapered part or layerscomprising second type fibres. The combined structure may be placed in avacuum enclosure and provided with vacuum prior to curing. Finally, thepre-form structure is cured.

FIG. 8 shows an example of a cross section of a pre-form that isenhanced for connection to a structure or other pre-forms beingreinforced by a second type fibres. More thorough connections areparticularly important when the physical properties of the structures tobe connected differ significantly. Typically, the physical propertiesare to a high extent dictated by the reinforcement fibres, and examplesof relevant physical properties are the coefficient of thermalexpansion, CTE, and Young's modulus. Hence, these types of connectionsare for example relevant when a composite comprising carbon fibres areconnected to a composite reinforced by another type of fibres, since theCTE of carbon fibres are very low and may even be negative. However, thesame type of connections may be used for strong connections betweencomposites reinforced by other types of fibres. The second type fibresmay be any of the fibre-types mentioned previously in the descriptionand for example this type of connection may be used for connecting acarbon fibre-reinforced composite to a glass fibre-reinforced composite.The pre-form in FIG. 8 has layers of second type fibres 62 (e.g., glassfibres) interlaced between the layers of carbon fibres 60 of the bulkpre-form.

In a preferred embodiment, the overlay distance of the interlace 64 isgreater than the extent of the end condition of the interfacial stressbetween layers having an appreciable amount of or exclusively containingcarbon fibres and layers having an appreciable amount of or exclusivelycontaining the second type of fibres, since this will prevent a couplingor build-up of stress between the interlaced layers. For the same reasonand for reason of introducing a safety margin, it is more preferred thatthe distance of the interlace 64 is greater than 2 times the extent ofthe end condition of the interfacial stress.

In another preferred embodiment, the distance 66 between the ends of thelayers having an appreciable amount of the second type fibres which areinterlaced between layers having an appreciable amount of carbon fibresare separated by a distance greater than the extent of the end conditionof the interfacial stress between layers having an appreciable amount ofcarbon fibres and layers having an appreciable amount of the second typefibres. Again, this is to prevent a coupling or build-up of stressbetween the interlaced layers. For the same reason and for reason ofintroducing a safety margin, it is more preferred that the distancebetween the ends of the layers having an appreciable amount of thesecond type fibres is greater than 2 times the extent of the endcondition of the interfacial stress.

In a preferred embodiment, the pre-form is further treated bypre-consolidation to form a pre-consolidated pre-form as described inthe following section. Pre-consolidation is particularly useful when thefibres are provided as individual fibres, fibre tows, fibre tow-pregscompared to fibres provided in prepregs as a lower viscosity during thepre-consolidation process. This will increase the redistribution ofresin and/or fibres, which is highly desirable as it increases thehomogeneity of the resulting product.

By pre-consolidation is herein meant a process, whereby gas inside apre-form is removed and a low porosity pre-form is produced.Pre-consolidation involves redistribution of a resin and optionally aredistribution of fibres. Furthermore, pre-consolidation may involve alimited curing of the resin. Pre-consolidation is particularly useful asit produces a dense pre-form (hereinafter named a pre-consolidatedpre-form). Pre-consolidated pre-forms and composites prepared frompre-consolidated pre-forms will be appreciated amongst others due togood reproducibility, low porosity, high homogeneity, high strength,ability to plastical shaping of the pre-consolidated pre-form, abilityto be connected to other pre-forms and/or other structures, suitabilityfor automation and long shelf life without premature curing.

When the pre-consolidation involves a limited curing, this limitedcuring may involve a release of up to 50% or the energy that will bereleased by a complete curing of the resin. However, it is preferredthat the extent of curing is limited to an extent that will allow thepre-form to be deformed plastically. The degree of curing that willallow for plastical deformation of a pre-consolidated pre-form dependsamongst others on the exact resin as well as on the fibre type and fibrecontent. Generally, it is preferred that the limited curing involvesless than about 20% of the energy that will be released by a completecuring of the resin and more preferably that the limited curing involvesbetween 3 to 15% of the energy that will be released by a completecuring.

Generally speaking, the pre-consolidation process should reduce theporosity of a pre-form. However, it is preferred that the resultingporosity of the pre-consolidated pre-form is less than 5% by volume,preferably less than 2% by volume and more preferably less than 1% byvolume. In some cases, a porosity of even 1% may reduce the propertiesof a composite considerably. In these cases, it will be appreciated thatthe method and the pre-consolidated pre-forms may be produced withporosities well below 1%. For example, a reproduced porosity of about0.2% by volume was realized for a composite with 60% carbon fibres inepoxy. The reduction of the porosity may for example be a result ofexposing the pre-form to a pressure and/or a vacuum in relation to thepre-consolidation process.

The porosity of the pre-consolidated pre-form can not be establisheddirectly, as a density is not known and may vary throughout thematerial. Hence, the porosity should be established by optical method ona materialographic sample. Preparation of materialographic samples froman uncured pre-consolidated pre-form is very demanding, since thematerial comprises both a very soft element (i.e. a resin) and a veryhard element (i.e. the fibre). To establish a reproducible result, it ishence necessary to cure the pre-form prior to materialographicpreparation. This curing should be pressureless to ensure that theporosity is unaffected by the process.

To ensure handleability, the pre-consolidated pre-form should besubstantially unsticky, i.e. it should be easily releasable from anyrelevant surface and it should not leave excessive amounts of resin on asurface when released.

To ensure a long shelf life and/or stability during transportation it isimportant that the curing reaction of the bulk of the resin issufficiently low at room temperature and that a catalyst—if present—isnot activated by accident. For example, if the catalyst is activated byheating, it should be ensured that the activation temperature isconsiderably higher than the expected maximum temperature duringstorage.

One of the features of the pre-consolidated pre-forms is that they areat least partially deformable. This may for example be realized throughthe balanced and limited curing during the pre-consolidation process. Ina possible use of the pre-form, at least a part of a pre-consolidatedpre-form is capable of being bent around an axis parallel to the mainfibre orientation with a diameter of more than 1 cm. However, in somecases a pre-consolidated pre-form may be bent with a diameter of morethan 5 cm by plastic deformation. The low bending diameters may berealized by rearranging of resin and/or fibres or by three-dimensionalforming of a pre-form. By three-dimensional forming is herein meant thatthe thickness (e.g., the number of layers or amount of fibres and/orresin) and/or the form of the ground plan is adjusted for a part of thepre-form relative to the bulk of the pre-form. Typically, only a part ofthe pre-consolidated pre-form is prepared for very sharp bending,whereas bending around an axis with larger diameters, e.g., 50 cm, mayoften be realized by all parts of the pre-consolidated pre-form.

The stiffness of a pre-form realized during a pre-consolidation processshould ensure that the pre-consolidated pre-form is stiff enough toprevent relaxation of the pre-consolidated pre-form in the lengthdirection of the fibres when placed on a non-flat surface and yet allowfor plastic deformation around an axis parallel to the length directionof the fibres. In particular, when a pre-consolidated pre-formcomprising carbon fibres is placed on crossing layers of glass fibres orglass fibre prepregs with partial overlay, then the pre-consolidatedpre-form should remain substantially flat during laying and curing,whereas the glass fibres should adjust to the shape/form of thepre-consolidated pre-form. Hence, the carbon fibres will remain straightleading to increased strength of the combined structure.

The pre-consolidation process often leads to an increase in viscosity ofthe resin in the pre-form, for example by a partial curing. It ispreferred that the viscosity at room temperature is increased by afactor of at least two and more preferably by a factor of at least five,as an increase in viscosity will enhance handleability, strength andunstickyness. In some cases, the viscosity may be increased by a muchhigher factor like for example 10, 100 or 1000. This is for example thecase if part of the resin is injected into the pre-form as a roomtemperature liquid. Another way to express the increase in viscosity isto look at viscosity directly. It is preferred that the viscosity of theresin in the unconsolidated pre-form is between about 100 to 10,000 cPat the temperature where the pre-consolidation process is conducted,preferably between about 500 to 3,000 cP.

The temperature where the pre-consolidation process is conducted mayvary considerably depending particularly on the composition of theresin. Typically, the pre-consolidation temperatures for epoxy-basedresin systems are 50 to 90° C. and preferably 60 to 80° C. However, bothhigher and lower temperatures may be feasible in some systems.

The pre-consolidation process may lead to an increase in the glasstransition temperature, T_(g), of the resin, for example by a partialcuring. It is preferred that the T_(g) of the resin is increased duringpre-consolidation by at least 2° C. and preferably by at least 5° C., asan increase in T_(g) usually indicates an increase in the averagemolecular weight of the resin, which will enhance handleability,strength and unstickyness. In some cases, T_(g) may be increased more.This is particularly the case when T_(g) of the unconsolidated pre-formis very low.

Generally speaking, a pre-consolidated pre-form according to theinvention with an epoxy-based resin system should typically have a T_(g)between −10 to +30° C. and preferably a T_(g) between −5 to 10° C. In apreferred embodiment, T_(g) of the resin of the pre-consolidatedpre-form is higher than about 0° C. and preferably higher than about 3°C. For the unconsolidated pre-form T_(g) of the resin should be belowabout 5° C. and preferably below about 2° C.

In some cases, curing of a pre-consolidated pre-form without beingexposed to a vacuum will result in a material with properties equivalentto a vacuum-cured pre-form, since porosity has been eliminated orgreatly reduced during the pre-consolidation process prior to thecuring.

The resin may comprise more than one system, for example the resin maycomprise two systems. These systems may be any combination of differentor the same type of systems. However, it is preferred that the resincomprises two substantially epoxy-based systems. The systems of a resinshould be compatible. In a preferred embodiment, two epoxy-based systemscomprise a common component. The common component may for example be acommon catalyst, a common amine component or a common epoxy component,however, it is preferred that the common component is an epoxycomponent. A resin comprising two epoxy-based systems with a commonepoxy component may comprise an amine component of a first epoxy-basedsystem that will react to the common epoxy component at a firstrelatively low temperature like for example below 50° C., preferablyabout room temperature. At this first temperature, a second epoxy-basedsystem is preferably non-reactive or the reaction takes place at a verylow rate. Since the reaction rate of the second epoxy-based systemshould be very low, it may advantageously be catalysed by a catalyst,which is un-active until activated. This activation may for example beby UV-light, addition of a compound or by heating, however, it ispreferred that the catalyst is activated by heating.

In a preferred method of pre-consolidating a pre-form, a pre-form isplaced on a reactor surface like for example a plate, a mould, etc. Itis preferred that the reactor surface is flat and that it will withstandheating and/or vacuum. Then a pressure is applied to the pre-form. Thepressure may be applied by a press or—preferably—a vacuum within avacuum enclosure. If a vacuum is used, then a vacuum enclosure should beobtained prior to pressing. The vacuum enclosure may for examplecomprise a vacuum bag or it may comprise a reactor surface and aflexible cover connected in a vacuum-tight way to the reactor surface.Gas may for example be evacuated through the reactor surface or throughan opening in the vacuum bag or flexible cover. The pre-consolidation isactivated. The activation may take place before and/or during and/orafter applying of pressure. The activation comprises a reduction of theviscosity of the resin. This may for example be realized by physicalmeans (e.g., heating, addition of solvent, pressure etc.) and/or by achemical reaction. During the pre-consolidation process, a limitedcuring may or may not take place. When the porosity has been reduced toa desired level or another object of the pre-consolidation is obtained,the pre-consolidation process is terminated. The termination may forexample be a result of exhaustion of a first resin system or cooling ofthe pre-consolidated pre-form to a temperature, where the curingreaction is sufficiently slow and/or the viscosity is sufficiently lowfor the pre-consolidated pre-form to achieve the stability needed forthe desired shelf life.

In a preferred embodiment, the pre-form to be pre-consolidated is havingat least one non-continuous layer of resin, through which gas may beremoved during the pre-consolidation process. Hence, the gas need not beremoved from the pre-form in a plane of a layer of resin or in a planeof a layer of fibres. The transportation distance and risk of havingtrapped gas inside the pre-consolidated pre-form is greatly reduced. Ina more preferred embodiment, all layers of resin—optionally, except froma layer on top of the top layer of fibres or below the bottom layer offibres—are non-continuous.

An example of a method for securing that gas may continuously be removedfrom the pre-form during pre-consolidation involves a gradual activationof the pre-consolidation process starting either from the center of thepre-form and moving towards the surfaces or from a side or edge andmoving through the pre-form. For example this may be realized by heatingfrom the reaction surface only, hence activating gradually from the sideof the pre-form in contact with the reaction surface or by controlledmicrowave heating, hence activating gradually from the inside of thepre-form and moving towards the surfaces.

Pre-forms according to the invention and provided by a method accordingto the invention are very useful for preparation of composite members bycuring. Particularly, the pre-forms are highly useful for preparation ofmembers for wind turbine blades and particularly in spars in a windturbine blade, since these composites fulfil the quality requirementsand reproducibility requirements.

When larger structures comprising pre-forms according to the inventionor pre-forms produced by a method according to the invention are to beprepared, this may follow a method wherein the pre-form is shaped atleast partially plastically. The pre-form may be connected to one ormore further pre-consolidated pre-forms and/or unconsolidated pre-formsbefore or after shaping to provide a larger structure. The pre-form mayalso be connected to other structures. It is preferred but not requiredthat the connections involve a tapered part or layers comprising asecond type of fibres. The combined structure may be placed in a vacuumenclosure and provided with vacuum prior to curing. Finally, thepre-form structure is cured.

The properties of a laminar structure having layers of oriented fibresdepend to a large extent on the distribution of the main elements of thestructure resin, fibres and porosity. The resin possesses a low strengthcompared to the fibres and may hence provide a route for crackpropagation through the structure, if too large layers of resin arepresent. Porosity may reduce the strength of the structure dramaticallybut the adversity depends on the size of pores, the shape and thedistribution, i.e. the effect of small, isolated spherical pores islimited, whereas larger pores positioned in the interface between resinand fibres may be fatal to the structure. It is hence vital to be ableto control the distribution of the elements.

The extent of redistribution depends i.a. on the viscosity of the resinduring the compaction process, i.e. the lower the viscosity, the easierthe redistribution of the elements. By utilizing a pre-consolidationprocess the viscosity of the resin may be lowered more than what isfeasible in the prior art, since the structure is not limited to supporta particular shape during the process. When the pre-consolidationinvolves a limited curing of the resin, the viscosity may be furtherreduced since the curing increases the handleability and reduces thesticking of the pre-consolidated pre-form. Hence, pre-consolidationallows for redistribution of resin and/or fibres to a much greaterextent than what may be realized in the prior art. The resultingpre-consolidated pre-forms may possess very low porosity as well as amore homogeneous structure. This may for example result in a compositestructure having a less pronounced laminar structure, i.e. where thelayers are less pronounced than a corresponding composite structurecomprising only pre-forms that were not pre-consolidated prior tocuring.

Table of Elements for Identification

-   2 Fibres indicating a main fibre orientation-   4 Resin-   4 a Line of resin-   4 b Dots or particles of resin-   4 c Random line of resin-   4 d Sheet of resin-   6 Adhesive-   6 a Adhesive near fibre initiation-   6 b Adhesive near fibre termination-   6 c Adhesive on central part of pre-form-   6 d Adhesive on central part of pre-form in zigzag pattern-   10 Ground plan of pre-form-   12 Through-going hole-   14 Direction of fibre laying-   γ Angle between edges of a ground plan of pre-form-   α Angle between edges of a ground plan of pre-form-   β Angle between edges of a ground plan of pre-form-   20 Tapered part of pre-form in the main direction of the fibres-   22 Tapered part of pre-form orthogonal to main direction of the    fibres-   24 Un-tapered part of pre-form-   30 a Amine of a first epoxy based system-   30 b Amine of a second epoxy based system-   32 Catalyst for a second epoxy based system-   34 Epoxy component-   36 Pre-mix comprising amine component and catalyst-   38 Mix and apply unit-   40 Mixed and distributed resin-   50 Ground plan of oriented fibres-   52 Part having a low number of layers of fibres-   54 Part having an intermediate number of layers of fibres-   56 Part having a high number of layers of fibres-   60 Fibre layer comprising carbon fibres-   62 Fibre layer comprising second type fibres-   64 Overlay distance of the interlace-   66 Distance between the ends of layers comprising second type fibres

1. A pre-form, said pre-form comprising: a resin and at least threelayers of oriented fibre tows formed of fibres composed of a firstmaterial, a part of said pre-form being formed three-dimensionally sothat said part of said pre-form is tapered, said pre-form adapted forenhanced coupling of said pre-form to a composite member comprisingsecond fibres composed of a second material which is different from thefirst material, a part of the pre-form, extending from a main portionthereof, comprising layers comprising at least an appreciable amount offibres composed of the second material.
 2. A pre-form according to claim1, wherein said pre-form comprises carbon fibres and said pre-form beingenhanced for coupling of said pre-form to a composite member comprisingsecond type fibres and a resin, said second type fibres selected fromthe group of glass fibres, aramid-fibres, synthetic fibres, bio fibres,mineral fibres, metal fibres or boron fibres, and a part of saidpre-form towards said composite member is provided with layers having anappreciable amount of, or exclusively containing said second type fibresinterlaced between layers having an appreciable amount of, orexclusively containing carbon fibres and said second type fibres extendbeyond said pre-form.
 3. The pre-form of claim 1, wherein the resincomprises a thermosetting resin.
 4. The pre-form of claim 1, wherein thefirst fibres comprise carbon fibres.
 5. The pre-form of claim 1, wherein the part of the pre-form extending from a main portion thereofcomprises almost exclusively fibres composed of the second material. 6.The pre-form of claim 1, wherein said pre-form comprises carbon fibresand said pre-form being enhanced for coupling of said pre-form to acomposite member said second type fibres selected from the group ofglass fibres, aramid-fibres, synthetic fibres, bio fibres, mineralfibres, metal fibres or boron fibres.
 7. The pre-form of claim 1,wherein said first fibres comprise carbon fibres and said second typefibres selected from the group of glass fibres, aramid-fibres, syntheticfibres, bio fibres, mineral fibres, metal fibres or boron fibres, and apart of said pre-form towards said composite member is provided withlayers having an appreciable amount of, or exclusively containing saidsecond type fibres interlaced between layers having an appreciableamount of, or exclusively containing carbon fibres and said second typefibres extend beyond said pre-form.
 8. A pre-form according to claim 1,wherein the second fibres are selected from the group consisting ofcarbon fibres, glass fibres, aramide fibres, synthetic fibres, biofibres, mineral fibres, metal fibres and boron fibres.
 9. A pre-formaccording to claim 1, wherein the fibres are continuous fibres.
 10. Apre-form according to claim 1, wherein the resin is mainly anepoxy-based resin or a polyester-based resin.
 11. A pre-form accordingto claim 10, wherein the resin comprises two resin systems.
 12. Apre-form according to claim 1, wherein the pre-form is substantiallyrectangular as observed in plan view from above looking down.
 13. Apre-form according to claim 12, wherein the distance between theparallel sides is at least 3 times the length of any of the parallelsides.
 14. A pre-form according to claim 13, wherein the pre-form issubstantially quadrangular or triangular in plan view as observed fromabove looking down.
 15. A pre-form according to claim 1, wherein thepre-form is substantially trapezoid when in plan view as viewed fromabove looking down.
 16. The pre-form of claim 15, wherein the trapezoidhas angles (α,β) which are substantially the same.
 17. A pre-formaccording to claim 1, wherein the fibres of each layer are orientedprimarily unidirectionally, respectively.
 18. A pre-form according toclaim 1, wherein the fibres are oriented substantially orthogonally toan edge of the pre-form.
 19. A pre-form according to claim 1, whereinthe fibres are oriented substantially parallel to an edge of thepre-form.
 20. A pre-form according to claim 1, wherein fibres areoriented to strengthen areas of the final element which will be exposedto higher stress during service.
 21. A pre-form according to claim 1,wherein orientation of fibres is substantially the same in all layers.